/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 */
package org.apache.tomcat.util.bcel.classfile;

import org.apache.tomcat.util.bcel.Const;

import java.io.DataInput;
import java.io.IOException;

/**
 * This class represents the constant pool, i.e., a table of constants, of
 * a parsed classfile. It may contain null references, due to the JVM
 * specification that skips an entry after an 8-byte constant (double,
 * long) entry.  Those interested in generating constant pools
 * programatically should see <a href="../generic/ConstantPoolGen.html">
 * ConstantPoolGen</a>.
 *
 * @see Constant
 */
public class ConstantPool {

	private final Constant[] constant_pool;

	/**
	 * Read constants from given input stream.
	 *
	 * @param input Input stream
	 * @throws IOException
	 * @throws ClassFormatException
	 */
	ConstantPool(final DataInput input) throws IOException, ClassFormatException {
		final int constant_pool_count = input.readUnsignedShort();
		constant_pool = new Constant[constant_pool_count];
	    /* constant_pool[0] is unused by the compiler and may be used freely
         * by the implementation.
         */
		for (int i = 1; i < constant_pool_count; i++) {
			constant_pool[i] = Constant.readConstant(input);
            /* Quote from the JVM specification:
             * "All eight byte constants take up two spots in the constant pool.
             * If this is the n'th byte in the constant pool, then the next item
             * will be numbered n+2"
             *
             * Thus we have to increment the index counter.
             */
			if (constant_pool[i] != null) {
				byte tag = constant_pool[i].getTag();
				if ((tag == Const.CONSTANT_Double) || (tag == Const.CONSTANT_Long)) {
					i++;
				}
			}
		}
	}

	/**
	 * Get constant from constant pool.
	 *
	 * @param index Index in constant pool
	 * @return Constant value
	 * @see Constant
	 */
	public Constant getConstant(final int index) {
		if (index >= constant_pool.length || index < 0) {
			throw new ClassFormatException("Invalid constant pool reference: " + index
					+ ". Constant pool size is: " + constant_pool.length);
		}
		return constant_pool[index];
	}

	/**
	 * Get constant from constant pool and check whether it has the
	 * expected type.
	 *
	 * @param index Index in constant pool
	 * @param tag   Tag of expected constant, i.e., its type
	 * @return Constant value
	 * @throws ClassFormatException If the constant is not of the expected type
	 * @see Constant
	 */
	public Constant getConstant(final int index, final byte tag) throws ClassFormatException {
		Constant c;
		c = getConstant(index);
		if (c == null) {
			throw new ClassFormatException("Constant pool at index " + index + " is null.");
		}
		if (c.getTag() != tag) {
			throw new ClassFormatException("Expected class `" + Const.getConstantName(tag)
					+ "' at index " + index + " and got " + c);
		}
		return c;
	}
}
